-- This is a vendored dependency.
-- Original Source: https://github.com/knyar/nginx-lua-prometheus
-- vim: ts=2:sw=2:sts=2:expandtab
--
-- This module uses a single dictionary shared between Nginx workers to keep
-- all metrics. Each counter is stored as a separate entry in that dictionary,
-- which allows us to increment them using built-in `incr` method.
--
-- Prometheus requires that (a) all samples for a given metric are presented
-- as one uninterrupted group, and (b) buckets of a histogram appear in
-- increasing numerical order. We satisfy that by carefully constructing full
-- metric names (i.e. metric name along with all labels) so that they meet
-- those requirements while being sorted alphabetically. In particular:
--
--  * all labels for a given metric are presented in reproducible order (the one
--    used when labels were declared). "le" label for histogram metrics always
--    goes last;
--  * bucket boundaries (which are exposed as values of the "le" label) are
--    presented as floating point numbers with leading and trailing zeroes.
--    Number of of zeroes is determined for each bucketer automatically based on
--    bucket boundaries;
--  * internally "+Inf" bucket is stored as "Inf" (to make it appear after
--    all numeric buckets), and gets replaced by "+Inf" just before we
--    expose the metrics.
--
-- For example, if you define your bucket boundaries as {0.00005, 10, 1000}
-- then we will keep the following samples for a metric `m1` with label
-- `site` set to `site1`:
--
--   m1_bucket{site="site1",le="0000.00005"}
--   m1_bucket{site="site1",le="0010.00000"}
--   m1_bucket{site="site1",le="1000.00000"}
--   m1_bucket{site="site1",le="Inf"}
--   m1_count{site="site1"}
--   m1_sum{site="site1"}
--
-- "Inf" will be replaced by "+Inf" while publishing metrics.
--
-- You can find the latest version and documentation at
-- https://github.com/knyar/nginx-lua-prometheus
-- Released under MIT license.

-- Default set of latency buckets, 5ms to 10s:
local DEFAULT_BUCKETS = {
    0.005, 0.01, 0.02, 0.03, 0.05, 0.075, 0.1, 0.2, 0.3,
    0.4, 0.5, 0.75, 1, 1.5, 2, 3, 4, 5, 10
}

-- Generate full metric name that includes all labels.
--
-- Args:
--   name: string
--   label_names: (array) a list of label keys.
--   label_values: (array) a list of label values.
-- Returns:
--   (string) full metric name.
local function full_metric_name(name, label_names, label_values)
    if not label_names then
        return name
    end
    local label_parts = {}
    for idx, key in ipairs(label_names) do
        local label_value = (string.format("%s", label_values[idx]):gsub("[^\032-\126]", "")) -- strip non-printable characters:gsub("\\", "\\\\"):gsub('"', '\\"'))
        table.insert(label_parts, key .. '="' .. label_value .. '"')
    end
    return name .. "{" .. table.concat(label_parts, ",") .. "}"
end

-- Metric is a "parent class" for all metrics.
local Metric = {}
function Metric:new(o)
    o = o or {}
    setmetatable(o, self)
    self.__index = self
    return o
end

-- Checks that the right number of labels values have been passed.
--
-- Args:
--   label_values: an array of label values.
--
-- Returns:
--   an error message or nil
function Metric:check_label_values(label_values)
    if self.label_names == nil and label_values == nil then
        return
    elseif self.label_names == nil and label_values ~= nil then
        return "Expected no labels for " .. self.name .. ", got " .. #label_values
    elseif label_values == nil and self.label_names ~= nil then
        return "Expected " .. #self.label_names .. " labels for " ..
                self.name .. ", got none"
    elseif #self.label_names ~= #label_values then
        return "Wrong number of labels for " .. self.name .. ". Expected " ..
                #self.label_names .. ", got " .. #label_values
    else
        for i, k in ipairs(self.label_names) do
            if label_values[i] == nil then
                return "Unexpected nil value for label " .. k .. " of " .. self.name
            end
        end
    end
end

local Counter = Metric:new()
-- Increase a given counter by `value`
--
-- Args:
--   value: (number) a value to add to the counter. Defaults to 1 if skipped.
--   label_values: an array of label values. Can be nil (i.e. not defined) for
--     metrics that have no labels.
function Counter:inc(value, label_values)
    local err = self:check_label_values(label_values)
    if err ~= nil then
        self.prometheus:log_error(err)
        return
    end
    if value ~= nil and value < 0 then
        self.prometheus:log_error_kv(self.name, value, "Value should not be negative")
        return
    end

    self.prometheus:inc(self.name, self.label_names, label_values, value or 1)
end

-- Delete a given counter
--
-- Args:
--   label_values: an array of label values. Can be nil (i.e. not defined) for
--     metrics that have no labels.
function Counter:del(label_values)
    local err = self:check_label_values(label_values)
    if err ~= nil then
        self.prometheus:log_error(err)
        return
    end
    self.prometheus:set(self.name, self.label_names, label_values, nil)
end

-- Delete all metrics for this counter. If this counter have no labels, it is
--   just the same as Counter:del() function. If this counter have labels, it
--   will delete all the metrics with different label values.
function Counter:reset()
    self.prometheus:reset(self.name)
end

local Gauge = Metric:new()
-- Set a given gauge to `value`
--
-- Args:
--   value: (number) a value to set the gauge to. Should be defined.
--   label_values: an array of label values. Can be nil (i.e. not defined) for
--     metrics that have no labels.
function Gauge:set(value, label_values)
    if value == nil then
        self.prometheus:log_error("No value passed for " .. self.name)
        return
    end
    local err = self:check_label_values(label_values)
    if err ~= nil then
        self.prometheus:log_error(err)
        return
    end
    self.prometheus:set(self.name, self.label_names, label_values, value)
end

-- Delete a given gauge
--
-- Args:
--   label_values: an array of label values. Can be nil (i.e. not defined) for
--     metrics that have no labels.
function Gauge:del(label_values)
    local err = self:check_label_values(label_values)
    if err ~= nil then
        self.prometheus:log_error(err)
        return
    end
    self.prometheus:set(self.name, self.label_names, label_values, nil)
end

-- Delete all metrics for this gauge. If this gauge have no labels, it is
--   just the same as Gauge:del() function. If this gauge have labels, it
--   will delete all the metrics with different label values.
function Gauge:reset()
    self.prometheus:reset(self.name)
end

-- Increase a given gauge by `value`
--
-- Args:
--   value: (number) a value to add to the gauge (a negative value when you
--     need to decrease the value of the gauge). Defaults to 1 if skipped.
--   label_values: an array of label values. Can be nil (i.e. not defined) for
--     metrics that have no labels.
function Gauge:inc(value, label_values)
    local err = self:check_label_values(label_values)
    if err ~= nil then
        self.prometheus:log_error(err)
        return
    end
    local key = full_metric_name(self.name, self.label_names, label_values)

    local newval, err = self.dict:incr(key, value)
    if newval then
        return
    end
    -- Yes, this looks like a race, so I guess we might under-report some values
    -- when multiple workers simultaneously try to create the same metric.
    -- Hopefully this does not happen too often (shared dictionary does not get
    -- reset during configuation reload).
    if err == "not found" then
        self:set_key(key, value)
        return
    end
    -- Unexpected error
    self:log_error_kv(key, value, err)
end

local Histogram = Metric:new()
-- Record a given value in a histogram.
--
-- Args:
--   value: (number) a value to record. Should be defined.
--   label_values: an array of label values. Can be nil (i.e. not defined) for
--     metrics that have no labels.
function Histogram:observe(value, label_values)
    if value == nil then
        self.prometheus:log_error("No value passed for " .. self.name)
        return
    end
    local err = self:check_label_values(label_values)
    if err ~= nil then
        self.prometheus:log_error(err)
        return
    end
    self.prometheus:histogram_observe(self.name, self.label_names, label_values, value)
end

local Prometheus = {}
Prometheus.__index = Prometheus
Prometheus.initialized = false

-- Construct bucket format for a list of buckets.
--
-- This receives a list of buckets and returns a sprintf template that should
-- be used for bucket boundaries to make them come in increasing order when
-- sorted alphabetically.
--
-- To re-phrase, this is where we detect how many leading and trailing zeros we
-- need.
--
-- Args:
--   buckets: a list of buckets
--
-- Returns:
--   (string) a sprintf template.
local function construct_bucket_format(buckets)
    local max_order = 1
    local max_precision = 1
    for _, bucket in ipairs(buckets) do
        assert(type(bucket) == "number", "bucket boundaries should be numeric")
        -- floating point number with all trailing zeros removed
        local as_string = string.format("%f", bucket):gsub("0*$", "")
        local dot_idx = as_string:find(".", 1, true)
        max_order = math.max(max_order, dot_idx - 1)
        max_precision = math.max(max_precision, as_string:len() - dot_idx)
    end
    return "%0" .. (max_order + max_precision + 1) .. "." .. max_precision .. "f"
end

-- Extract short metric name from the full one.
--
-- Args:
--   full_name: (string) full metric name that can include labels.
--
-- Returns:
--   (string) short metric name with no labels. For a `*_bucket` metric of
--     histogram the _bucket suffix will be removed.
local function short_metric_name(full_name)
    local labels_start, _ = full_name:find("{")
    if not labels_start then
        -- no labels
        return full_name
    end
    local suffix_idx, _ = full_name:find("_bucket{")
    if suffix_idx and full_name:find("le=") then
        -- this is a histogram metric
        return full_name:sub(1, suffix_idx - 1)
    end
    -- this is not a histogram metric
    return full_name:sub(1, labels_start - 1)
end

-- Makes a shallow copy of a table
local function copy_table(table)
    local new = {}
    if table ~= nil then
        for k, v in ipairs(table) do
            new[k] = v
        end
    end
    return new
end

-- Check metric name and label names for correctness.
--
-- Regular expressions to validate metric and label names are
-- documented in https://prometheus.io/docs/concepts/data_model/
--
-- Args:
--   metric_name: (string) metric name.
--   label_names: label names (array of strings).
--
-- Returns:
--   Either an error string, or nil of no errors were found.
local function check_metric_and_label_names(metric_name, label_names)
    if not metric_name:match("^[a-zA-Z_:][a-zA-Z0-9_:]*$") then
        return "Metric name '" .. metric_name .. "' is invalid"
    end
    for _, label_name in ipairs(label_names or {}) do
        if label_name == "le" then
            return "Invalid label name 'le' in " .. metric_name
        end
        if not label_name:match("^[a-zA-Z_][a-zA-Z0-9_]*$") then
            return "Metric '" .. metric_name .. "' label name '" .. label_name ..
                    "' is invalid"
        end
    end
end

-- Initialize the module.
--
-- This should be called once from the `init_by_lua` section in nginx
-- configuration.
--
-- Args:
--   dict_name: (string) name of the nginx shared dictionary which will be
--     used to store all metrics
--   prefix: (optional string) if supplied, prefix is added to all
--   metric names on output
--
-- Returns:
--   an object that should be used to register metrics.
function Prometheus.init(dict_name, prefix)
    local self = setmetatable({}, Prometheus)
    dict_name = dict_name or "prometheus_metrics"
    self.dict = ngx.shared[dict_name]
    if self.dict == nil then
        ngx.log(ngx.ERR,
            "Dictionary '", dict_name, "' does not seem to exist. ",
            "Please define the dictionary using `lua_shared_dict`.")
        return self
    end
    -- by default resty_counter fallback to shdict
    self.resty_counter = self.dict
    self.help = {}
    if prefix then
        self.prefix = prefix
    else
        self.prefix = ''
    end
    self.type = {}
    self.registered = {}
    self.buckets = {}
    self.bucket_format = {}
    self.initialized = true

    self:counter("nginx_metric_errors_total",
        "Number of nginx-lua-prometheus errors")
    self.dict:set("nginx_metric_errors_total", 0)
    return self
end

-- enable the use the lua-resty-counter for Counter and Histogram
function Prometheus:set_resty_counter(counter)
    self.resty_counter = counter
end

function Prometheus:log_error(...)
    ngx.log(ngx.ERR, ...)
    self.dict:incr("nginx_metric_errors_total", 1)
end

function Prometheus:log_error_kv(key, value, err)
    self:log_error("Error while setting '", key, "' to '", value, "': '", err, "'")
end

-- Register a counter.
--
-- Args:
--   name: (string) name of the metric. Required.
--   description: (string) description of the metric. Will be used for the HELP
--     comment on the metrics page. Optional.
--   label_names: array of strings, defining a list of metrics. Optional.
--
-- Returns:
--   a Counter object.
function Prometheus:counter(name, description, label_names)
    if not self.initialized then
        ngx.log(ngx.ERR, "Prometheus module has not been initialized")
        return
    end

    local err = check_metric_and_label_names(name, label_names)
    if err ~= nil then
        self:log_error(err)
        return
    end

    if self.registered[name] then
        self:log_error("Duplicate metric " .. name)
        return
    end
    self.registered[name] = true
    self.help[name] = description
    self.type[name] = "counter"

    return Counter:new { name = name, label_names = label_names, prometheus = self }
end

-- Register a gauge.
--
-- Args:
--   name: (string) name of the metric. Required.
--   description: (string) description of the metric. Will be used for the HELP
--     comment on the metrics page. Optional.
--   label_names: array of strings, defining a list of metrics. Optional.
--
-- Returns:
--   a Gauge object.
function Prometheus:gauge(name, description, label_names)
    if not self.initialized then
        ngx.log(ngx.ERR, "Prometheus module has not been initialized")
        return
    end

    local err = check_metric_and_label_names(name, label_names)
    if err ~= nil then
        self:log_error(err)
        return
    end

    if self.registered[name] then
        self:log_error("Duplicate metric " .. name)
        return
    end
    self.registered[name] = true
    self.help[name] = description
    self.type[name] = "gauge"

    return Gauge:new { name = name, label_names = label_names, prometheus = self }
end

-- Register a histogram.
--
-- Args:
--   name: (string) name of the metric. Required.
--   description: (string) description of the metric. Will be used for the HELP
--     comment on the metrics page. Optional.
--   label_names: array of strings, defining a list of metrics. Optional.
--   buckets: array if numbers, defining bucket boundaries. Optional.
--
-- Returns:
--   a Histogram object.
function Prometheus:histogram(name, description, label_names, buckets)
    if not self.initialized then
        ngx.log(ngx.ERR, "Prometheus module has not been initialized")
        return
    end

    local err = check_metric_and_label_names(name, label_names)
    if err ~= nil then
        self:log_error(err)
        return
    end

    for _, suffix in ipairs({ "", "_bucket", "_count", "_sum" }) do
        if self.registered[name .. suffix] then
            self:log_error("Duplicate metric " .. name .. suffix)
            return
        end
        self.registered[name .. suffix] = true
    end
    self.help[name] = description
    self.type[name] = "histogram"

    self.buckets[name] = buckets or DEFAULT_BUCKETS
    self.bucket_format[name] = construct_bucket_format(self.buckets[name])

    return Histogram:new { name = name, label_names = label_names, prometheus = self }
end

-- Set a given dictionary key.
-- This overwrites existing values, so it should only be used when initializing
-- metrics or when explicitely overwriting the previous value of a metric.
function Prometheus:set_key(key, value)
    local ok, err = self.dict:safe_set(key, value)
    if not ok then
        self:log_error_kv(key, value, err)
    end
end

-- Increment a given metric by `value`.
--
-- Args:
--   name: (string) short metric name without any labels.
--   label_names: (array) a list of label keys.
--   label_values: (array) a list of label values.
--   value: (number) value to add (a negative value when you need to decrease
--     the value of the gauge). Optional, defaults to 1.
function Prometheus:inc(name, label_names, label_values, value)
    local key = full_metric_name(name, label_names, label_values)
    if value == nil then value = 1 end

    self.resty_counter:incr(key, value)
end

-- Set the current value of a gauge to `value`
--
-- Args:
--   name: (string) short metric name without any labels.
--   label_names: (array) a list of label keys.
--   label_values: (array) a list of label values.
--   value: (number) the new value for the gauge.
function Prometheus:set(name, label_names, label_values, value)
    local key = full_metric_name(name, label_names, label_values)
    self:set_key(key, value)
end

-- Record a given value into a histogram metric.
--
-- Args:
--   name: (string) short metric name without any labels.
--   label_names: (array) a list of label keys.
--   label_values: (array) a list of label values.
--   value: (number) value to observe.
function Prometheus:histogram_observe(name, label_names, label_values, value)
    self:inc(name .. "_count", label_names, label_values, 1)
    self:inc(name .. "_sum", label_names, label_values, value)

    -- we are going to mutate arrays of label names and values, so create a copy.
    local l_names = copy_table(label_names)
    local l_values = copy_table(label_values)

    -- Last bucket. Note, that the label value is "Inf" rather than "+Inf"
    -- required by Prometheus. This is necessary for this bucket to be the last
    -- one when all metrics are lexicographically sorted. "Inf" will get replaced
    -- by "+Inf" in Prometheus:collect().
    table.insert(l_names, "le")
    table.insert(l_values, "Inf")
    self:inc(name .. "_bucket", l_names, l_values, 1)

    local label_count = #l_names
    for _, bucket in ipairs(self.buckets[name]) do
        if value <= bucket then
            -- last label is now "le"
            l_values[label_count] = self.bucket_format[name]:format(bucket)
            self:inc(name .. "_bucket", l_names, l_values, 1)
        end
    end
end

-- Delete all metrics in a gauge or counter. If this gauge or counter have labels, it
--   will delete all the metrics with different label values.
function Prometheus:reset(name)
    local keys = self.dict:get_keys(0)
    for _, key in ipairs(keys) do
        local value, err = self.dict:get(key)
        if value then
            local short_name = short_metric_name(key)
            if name == short_name then
                self:set_key(key, nil)
            end
        else
            self:log_error("Error getting '", key, "': ", err)
        end
    end
end

-- Prometheus compatible metric data as an array of strings.
--
-- Returns:
--   Array of strings with all metrics in a text format compatible with
--   Prometheus.
function Prometheus:metric_data()
    if not self.initialized then
        ngx.log(ngx.ERR, "Prometheus module has not been initialized")
        return
    end

    local keys = self.dict:get_keys(0)
    -- Prometheus server expects buckets of a histogram to appear in increasing
    -- numerical order of their label values.
    table.sort(keys)

    local seen_metrics = {}
    local output = {}
    for _, key in ipairs(keys) do
        local value, err = self.dict:get(key)
        if value then
            local short_name = short_metric_name(key)
            if not seen_metrics[short_name] then
                if self.help[short_name] then
                    table.insert(output, string.format("# HELP %s%s %s\n",
                        self.prefix, short_name, self.help[short_name]))
                end
                if self.type[short_name] then
                    table.insert(output, string.format("# TYPE %s%s %s\n",
                        self.prefix, short_name, self.type[short_name]))
                end
                seen_metrics[short_name] = true
            end
            -- Replace "Inf" with "+Inf" in each metric's last bucket 'le' label.
            if key:find('le="Inf"', 1, true) then
                key = key:gsub('le="Inf"', 'le="+Inf"')
            end
            table.insert(output, string.format("%s%s %s\n", self.prefix, key, value))
        else
            self:log_error("Error getting '", key, "': ", err)
        end
    end
    return output
end

-- Present all metrics in a text format compatible with Prometheus.
--
-- This function should be used to expose the metrics on a separate HTTP page.
-- It will get the metrics from the dictionary, sort them, and expose them
-- aling with TYPE and HELP comments.
function Prometheus:collect()
    ngx.header.content_type = "text/plain"
    ngx.print(self:metric_data())
end

return Prometheus
